Some Effects of Ultraviolet-B (UV-B) Radiation on the
Biosphere
Human health professionals and biological scientists would love to be
able to demonstrate a direct correlation between the amount of exposure
to UV-B radiation and the harm it causes. This is an enormously
complicated question that depends on many different variables, such as
varying degrees of susceptibility among different species, and most of
these variables are not yet completely understood. For example, the
same organism in different bodies of water in different parts of the
ocean may respond differently to UV-B increases. Furthermore, stress to
organisms and ecosystems from increased exposure to UV-B is modified by
interactions among many other stresses, such as lack of water or
nutrients. We live in a complex biosphere.

We know that increased exposure to UV-B radiation has specific
effects on human health, crops, terrestrial ecosystems, aquatic
ecosystems, and biogeochemical cycles. (Biogeochemical cycles refers
to the cycling of chemicals such as carbon and energy throughout the
Earth system.) This article will touch briefly on these effects, then
will explain what determines how much UV we are getting and how we
know.

The effects of UV-B radiation on human skin are varied and
widespread. UV-B induces skin cancer by causing mutation in DNA and
suppressing certain activities of the immune system. The United Nations
Environment Program estimates that a sustained 1 percent depletion of
ozone will ultimately lead to a 2-3 percent increase in the incidence of
non-melanoma skin cancer. UV-B may also suppress the bodys immune
response to Herpes simplex virus and to skin lesion development, and may
similarly harm the spleen.

Our hair and clothing protect us from UV-B, but our eyes are
vulnerable. Common eye problems resulting from over-exposure to UV-B
include cataracts, snow blindness, and other ailments, both in humans
and animals. While many modern sunglasses offer some UV protection, a
significant amount of UV can still reach our eyes in a high exposure
situation.

With regard to plants, UV-B impairs photosynthesis in many species.
Overexposure to UV-B reduces size, productivity, and quality in many of
the crop plant species that have been studied (among them, many
varieties of rice, soybeans, winter wheat, cotton, and corn). Similarly,
overexposure to UV-B impairs the productivity of phytoplankton in
aquatic ecosystems. UV-B increases plants susceptibility to
disease. Scientists have found it affects enzyme reactions that conduct
fundamental biological functions, it impairs cellular division in
developing sea urchin eggs, and it changes the movements and orientation
of tiny organisms as they move through ocean waters. Since some species
are more vulnerable to UV-B than others, an increase in UV-B exposure
has the potential to cause a shift in species composition and diversity
in various ecosystems. Because UV-B affects organisms that move
nutrients and energy through the biosphere, we can expect changes in
their activities to alter biogeochemical cycles. For example, reducing
populations of phytoplankton would significantly impact the worlds
carbon cycle, because phytoplankton store huge amounts of carbon in the
ocean.

Much of scientists work to determine the effects of increased
UV-B on the marine biosphere has focused around Antarctica because the
stratospheric ozone depletion there has been so dramatic, and because phytoplanktonwhich grow in
abundance around Antarcticaform the basis of the marine food
chain. Largely because of phytoplankton, oceans are responsible for the
production of at least half of the organic material in the
biosphere.

Exposure to ultraviolet radiation in
Antarctica is commonly highest in spring. (Image courtesy of NOAA)

In the Antarctic, increased exposure to UV-B radiation due to the
appearance of the ozone hole commonly results in at least a 6-12 percent
reduction in photosynthesis by phytoplankton in surface waters. In a
study of California coastal waters, effects of current levels of UV-B
radiation compared to historical levels range from 40 percent reduction
of photosynthesis by phytoplankton to a 10 percent increase. In fact,
phytoplankton off the California coast sometimes turn out to be more
susceptible to UV-B radiation than phytoplankton in Antarctica, to the
surprise of biologists.

Communities of plants, animals, and microorganisms may be more
resilient than we yet know. In spite of increased ultraviolet exposure
in Antarctica over the last decade or so, no catastrophic events have
occurred at the ecosystem level. However, the reason for this may be
that the large ozone hole lasts only from September to December and
covers a small geographic region relative to the entire globe. If the
ozone hole should remain for longer time periods, or if ozone were to be
reduced over a wider area every year, sooner or later, we could expect
to see major ecosystem changes. So many studies in both the laboratory
and the field have demonstrated serious consequences of increased UV-B
radiation on the biosphere that we need to improve our understanding of
the complex Earth environment and its responses to that radiation.

Overexposure to ultraviolet
radiation can change the flowering times of some kinds of plants and
therefore will affect the animals that depend on them. (Photograph courtesy Jeannie Allen)